The Breast Cancer Site
is real and money paid by site advertisers does indeed go towards providing free mammograms. When a visitor clicks on the "Fund Free Mammograms" button on the site, another page opens that contains banner advertisements from site sponsors. All of the fees paid by these sponsors are used to fund free mammograms.
I must admit that initially I thought this was a hoax, but I checked it at several urban legend and hoax sites. I keep the email about this in my Inbox, and periodically click the link. It's one of those things you can do in 30 seconds to help others. That's not too much to ask.